Enhancement of biological nitrogen removal from wastewater by low-intensity ultrasound

This paper investigated the influence of low-intensity ultrasound in biological nitrification and denitrification. The results showed that the nitrification activity of activated sludge could not be promoted significantly by ultrasound in 5-40 min with intensities ranging from 0.1 to 1.2 W∈cm^-2. It suggested the fact that nitrifying bacteria were insensitive to ultrasound, possibly related with their specific structures of cell membrane and ways of metabolism. Whereas, biological denitrification was enhanced quite remarkably by ultrasound and the optimal results were achieved at the ultrasonic intensity of 0.2 W∈cm^-2 and the irradiation time of 10 min. Compared with the control without ultrasonic irradiation, it took 5 h for the enhancement of denitrification rates induced by the optimum ultrasound to reach its peak level (16%). Therefore, ultrasound with intensity of 0.2 W∈cm^-2 could be employed in the biological denitrification system for 10 min every 5 h to obtain the optimal effect theoretically.